The present invention relates to fiber optic communication, and more particularly relates to modulation of light for use with fiber optics.
Present day fiber optic communication includes the use of light emitting semiconductors whose outputs are linked to fiber optic cables. Data transmission is controlled by switching the drive current in the semiconductor from above the emission threshold to below the emission threshold, thereby causing the light output of the semiconductor to be switched on and off, or high and low, respectively.
When a high current--above the emission threshold--is passed through the the semiconductor, optical output is high (on) and is used to represent the logical or binary datum "1". When the current through the semiconductor is low--below the emission threshold--optical output is low (off) and is used to represent the logical or binary datum "0".
A limiting factor of the data transmission rate in such a system is the speed at which the semiconductor reaches high optical output after the drive current has switched above the emission threshold, referred to as the rise time. Conversely, the speed at which the semiconductor's optical output turns off (low) after the drive current switches below the emission threshold, the fall time, also limits the rate of transmission. Normally, the fall time is longer than the rise time.
A typical value of a rise time for a light emitting semiconductor is on the order of 1 nanosecond (nS). Thus, taking into account the fall time and pulse width, the useable bandwidth of a fiber optic link is on the order of less than about 1000 Megahertz (MHz). An optical fiber can support bandwidths on the order of the frequency of the light used which is approximately 300 Terahertz (THz).
Another factor which limits fiber optic data transmission efficiency is fiber optic dispersion. Each time a laser emitting semiconductor is switched on--millions of times per second--transient turn-on characteristics of the particular semiconductor cause the temporary emission of a wider range of wavelengths, referred to as delta lambda, than is useful for fiber optic transmission. Consequently, the transmission rate must be slow enough for the transient to settle each time the semiconductor is switched on so that a distinct pulse can be transmitted.
FIG. 1 is a wave diagram showing switching optical output power 1 in a conventional fiber optic link controlled by drive current 2 of a modulation circuit 5 shown in FIG. 2. Low drive current results in low optical output and is used to represent a logical 0. High drive current results in high optical output and is used to represent a logical 1. The speed at which the semiconductor can be switched is limited partly by the rise time, or turn on speed 4. Another limiting factor is the response time 3 of the optical output power 1, which is on the order of 100 picoseconds (pS). The optical output power does not reach the high output until time 3 after the drive current 2 reaches its peak level.
FIG. 2 is a schematic diagram which shows light emanating from the output side 6 of a typical semiconductor light source in the "on" state, which is modulated by drive current 2 provided by modulation circuit 5.
U.S. Pat. No. 4,744,625 issued May 17, 1988 to Lanzisera for "Methods of and Apparatus for Providing Frequency Modulated Light" discloses a laser reflecting medium that is acoustically oscillated whereby reflected light reinforces or cancels light emanating from a companion source.
U.S. Pat. No. 4,768,852 issued Sep. 6, 1988 to Ih for "Apparatus for Optical Fiber Communication Using Travelling Wave Acousto-Optical Modulator and Injection Locked Lasers" discloses an apparatus which utilizes a laser beam split into multiple beams and also discloses a multiplexing scheme compatible with a multiple beam system to increase digital bit rates.
U.S. Pat. No. 5,015,619 issued May 14, 1991 to Wang for "Superconducting Mirror for Laser Gyroscope" discloses a superconductive mirror assembly whose reflectivity can be decreased, thereby allowing detection and measurement of the light transmitted through the superconductor leading to the calculation of rotational motion based on gyroscopic properties of the disclosed invention, after which the superconductor is returned to its reflective state.
IBM Technical Disclosure Bulletin Vol. 21 No. 11, April 1979, pp. 4715-4716, discloses a weak link grating structure fabricated in a superconducting transmission line and suggests that it could be used to modulate light using Josephson circuitry in conjunction with current pulses through the transmission line which acts to diffract, i.e., generate a new beam direction for, an incident light beam.